Maybe you’ve seen a little story today about mercury, pregnancy, fish, and attention-deficit/hyperactivity disorder (ADHD). If you haven’t, you probably will. The news release headline says, “Prenatal mercury exposure may be associated with risk of ADHD-related behaviors.” The modifiers in there “may be” and “related” won’t stop the mental math from this study translating as “pregnant+mercury=ADHD,” and it won't stop headlines from implying as much. Mix in a confusing finding about eating fish during pregnancy--protective or harmful?--and what’s a science consumer to do?

The study relied on a cohort of children born from 1993 to 1998 and their mothers. About 10 days after each mother gave birth, she filled out a questionnaire about what she’d eaten during pregnancy--including fish broken down by category--and gave up a hair sample. Investigators at Brigham and Women’s Hospital in Boston evaluated the hair for levels of methylmercury, an organic form of mercury that the central nervous system readily takes up. When the children reached the age of 8, they underwent testing that evaluated behaviors related to ADHD. The study was originally designed to assess the relationship, if any, between exposure to polychlorinated biphenyls contaminating a local harbor and neurodevelopment.

According to the findings, ADHD-related behaviors in the 607 evaluated 8-year-olds increased with increasing methylmercury values from mom’s hair around the time she gave birth. These associations were stronger for boys than girls for some features, with a slightly increased risk for ADHD-related behaviors above a mercury value of 1 microgram/gram in the hair. But fish consumption during pregnancy was linked to reduced levels of ADHD-related behaviors in the children.

Pregnancy and fish

What’s a pregnant woman to do, given that a lot of fish we’d like to eat contain a lot of mercury? “Fish is the predominant source of mercury intake for pregnant women,” says Bruce Lanphear, who wrote a commentary accompanying the study, both published in Archives of Pediatrics and Adolescent Medicine. That information, he adds, “shouldn’t change the recommendations to pregnant women about eating fish that is low in mercury.” Which fish are which? Here’s a handy guide, but briefly, salmon, trout, and catfish tend to be low mercury while shark, tuna, and mackerel (sob!) are no-nos.

So, first take-home is, you can keep eating fish. Just don’t eat fish that have a lot of mercury in them. Nothing’s changed there. In fact, eating fish without a lot of mercury seems to be protective, so get thee to a good fish market and pick something tasty, low-mercury, and sustainable for dinner. What we need here is a sustainable × low-mercury guide to make all of this easier.

ADHD and mercury

But what about this ADHD and mercury link? Well, this study doesn’t actually report a link between mercury and ADHD. As study senior author Susan Korrick, assistant professor of medicine at Brigham and Women’s Hospital in Boston, notes, “it’s really important to keep in mind that we did not look clinically at children diagnosed with ADHD.” This cohort was just a general population of children tracked after being born healthy and vaginally, and as with any children, no one knew when they were born how they’d turn out. So some might have ADHD, but overall, they simply represent a general population. “We were looking at behaviors largely across the normal range, which is very different from looking at a clinical diagnosis of ADHD,” Korrick says. “It’s beyond the scope of this research to make inferences about clinical ADHD.” ADHD has a large inherited component [PDF], so if your child has ADHD, you don’t need to waste hours guiltily assessing your fish intake during pregnancy.